[masraum]

Torsion bars are the 911 equivalent to springs. They are primarily to suspend the car and absorb the irregularities of the road as Jack said. They may resist roll to an extent because the outside springs don't want to compress, but at the same time the inside springs want to extend. So while one side is trying to stay flat the other side is actually fighting it somewhat.
Anti-roll bars actually resist rotation around the middle longitudinal axis of the car. When the outside tire/hub compresses the spring and is pushed up into the wheel well the end of the sway bar on that side is rotated up with the wheel. Since the bar extends over to the other side of the car that side, the inside, will also try to rotate up which is the opposite of what the spring is trying to do to the inside wheel. So the anti-roll bar is actually trying to lift the inside wheel and compress that inside spring. Since the weight of the car and the inside spring are resisting that rotation it will also be resisted on the outside wheel. The anti-roll bars are actually trying to keep the car flat while each spring doesn't really car what the other springs are doing as long as he gets to extend as much as he is able.

I'm certainly no expert, but from what I've heard and read I would go with the new torsion bars, drive the car a while like that, and then once you are really comfortable with the new torsion bars then change out the sway bars. Gradual changes instead of everything all at once.